Heat recovery from gas turbines is a common method for providing heat for the production of steam and hot water on offshore installations. Often there is a demand for large quantities of heat for use in the oil production or for other heat demands. Lately one has also explored the possibilities of recuperating heat for producing steam for injection into gas turbines in order to reduce NOx emissions to the atmosphere.
The heat recovery usually takes place by letting the warm exhaust from the gas turbine flow across a tube heat exchanger in order for the heat from the exhaust to be transferred to a liquid or steam based system.
Common to all these heat recovery units is that they are very large and heavy. A typical unit for hot water generation weighs about 50 tons and is 10×8×14 meters (width×length×height). This is normally not a problem on land where there often is enough space, but it can be a problem on offshore installations where the space is limited. This is particularly true for existing platforms experiencing a demand for additional heat.
The reason for these units being so large is generally the need for regulating the amount of exhaust to be supplied to the heat recovery unit. It is therefore often necessary to have a bypass system which is very space consuming. Today it is customary to regulate the amount of exhaust supply by means of a damper. Either the damper is of the one-blade type so that; as seen from the gas turbine, there will always be an open exhaust path, or one set of dampers is installed in the main conduit and a second set is installed in the bypass conduit, said sets working together. The disadvantage of this latter solution is that faulty operation may result in a blocked outlet as seen from the gas turbine.
In addition to the heat exchanger, a typical heat recovery unit will comprise a muffler, a regulating valve, a bypass system, a heat exchanger housing and an outlet chimney.
U.S. Pat. No. 6,302,191 suggests a different solution which has an integral bypass conduit. In this case the heat exchanger is arranged in an annular space which constitutes part of the exhaust conduit. The annular space is delimited on the inner side by a cylindrical valve slide, which concurrently forms the outer limitation of the bypass conduit. The valve slide is movable axially between two extreme positions. In the upper position, the valve slide opens for exhaust to the heat exchanger and closes the bypass, and in the lower position it opens the bypass and closes to the heat exchanger. In middle positions the exhaust is divided between the heat exchanger and the bypass.
In an offshore plant, the cylindrical valve slide according to U.S. Pat. No. 6,302,191 would be about 3 meters in diameter and 5-10 meters long. It would have to be supported so that it could be moved axially in a simple manner since the entire regulation of the exhaust is based on the axial movement of the cylinder. This necessitates slide or roller bearings for minimizing friction, and also necessitates that the wall thickness is large enough to avoid deflections during movements of the cylinder and due to repeated heating and cooling (operational temperatures are typically 600° C.). This makes the cylinder large and heavy. It must also be supported by complicated bearings which will be located in the hot exhaust, a situation which in similar applications has caused problems. The bearings are difficult to access without dismantling the entire unit.